[0001] The invention relates to a flame retardant polyethylene terephthalate moulding composition
which contains as flame retardant additive a complex salt of oxalic acid, and to the
products manufactured therefrom.
[0002] These moulding compositions are suitable to be processed into differently shaped
mouldings by injection moulding, extrusion, rotation moulding and the like.
[0003] The use of particular complex salts of oxalic acid for making a polyethylene terephthalate
composition flame retardant is known from the Netherlands Patent Application 7 612
884. In it it is illustrated that the use of such special oxalic acid salts has great
advantages over the use of the commonly employed flame retardant substances based
on halogens, phosphorus and nitrogen, the halogen compounds being the most important
of these.
[0004] In addition to being satisfactorily flame retardant, it offers the advantages of
the absence of considerable formation of smoke upon combustion, satisfactory processability
and non-toxicity.
[0005] Although polyethylene terephthalate (PETP) moulding compasitions may be rendered
fire resistant and flame retardant by adding particular complex salts of ox cids,
there is a general need for these properties to be further improved without detracting
he favourable mechanical properties of the objects made from the moulding composition.
[0006] Particularly, there is need for PETP-based moulding compositic vhich satisfy the
most rigid standards of incombustibility. A generally accepted test method is Jescribed
in Bulletin 94 of Underwriter's Laboratories (UL 94). According to this test method
a material is classified UL 94-VO, which is the best rating of incombustibility, if
a standard test bar extinguishes itself within on average 5 seconds after removal
of the test flames and does not drip flaming particles that ignite the dry absorbent
surgical cotton located below the test bar.
[0007] The same results are to be obtained in a test carried out with artifically aged material.
A more detailed rendering of the test method will be given hereinafter in the description.
[0008] United States Patent Specification 3 671 487 reveals glass fibre-reinforced PETP
- moulding compounds which contain a flame retardant agent and 0,9―1,9% by weight
of polytetrafluoroethylene (PTFE), based on the total composition. The test method
mentioned in said Patent Specification differs from the current UL 94 method, especially
in that no aged material has been examined, which generally implies that the material
has been less severely tested. For, it is often found that test bars which did meet
the VO standard before aging, no longer meet the flammability requirement after aging.
Moreover, according to said test method use was made of test bars having a thickness
of 3,2 mm (1/8"). The highest rating of non-burning can only be attained with the
thinner test bars of 1,6 mm (1/16") and 0,8 mm (1/32' which are more difficult to
be rendered incombustible.
[0009] Some moulding compounds do not le: themselves to being injection moulded into test
bars having a thickness of 0,8 mm (1/32"), the classification UL 94 VO for 1,6 mm
(1/16") being the highest attainable rating then.
[0010] Now it is known that the presence of a fibrous material, such as glass fibres, in
a polymer moulding compound containing a flame retardant agent will contribute to
the effectiveness thereof in that it inhibits the dripping of a burning object. Consequently,
objects made from polyester moulding compounds which do not contain fibrous material
such as glass fibres can hardly, if at all, be made to meet the highest requirements
of the UL 94 test for thickness values of 1,6 mm (1/16") and 0,8 mm (1/32"), unless
a very high proportion of flame retardant agent, usually more than 15% by weight,
is incorporated. Such high proportions of flame retardant agent, however, have a very
detrimental effect on the mechanical properties, especially the impact strength, so
that the resulting material will in fact not find application.
[0011] German Patent Application 2 433 966 describes a series of experiments with polybutylene
terephthalate (PBTF) moulding compounds which contain 12-21 % by weight-of flame retardant
agent consisting of a mixture of a halogen compound and antimony trioxide, and 0,5―1,0%
by weight of polytetrafluoroethylene ((PTFE), to which 0,5―1,0% by weight of non-combustible
potassium titanate fibres has been added. It is stated that without the addition of
these fibres the 3,2 mm (1/8") test bars will drip upon being subjected to the flammability
test and the material does not qualify for the UL 94 VO rating for 3,2 mm (1/8").
Not until the fibres have been added can this material be classed UL 94 VO for 3,2
mm (1/8").
[0012] Netherlands Patent Application 7 603 771 describes a series of experiments with PBTP
moulding compounds which contain 18% by weight of a flame retardant agent consisting
of a mixture of a halogen compound and 0,1―4,0% by weight of polytetrafluoroethylene
(PTFE), of which the test bars are classed UL 94 VO for 1,6 mm (1/16"). To that end,
however, an excessively large proportion of flame retardant agent is required. The
experiments also show that the incorporation into the PBTP moulding compound of 30
wt.% glass fibre in the absence of PTFE but in the process of 18 wt.% flame retardant
agent will also make the material meet the UL 94 VO requirement for 1,6 mm (1/16").
[0013] The present invention has for its object to provide a PETP moulding composition which
contains no or very little fibrous material, and satisfies the highest requirement
of flammability in accordance with the UL 94 VO test for the thicknesses: 3,2 mm,
1,6 mm and 0,8 mm (1/8", 1/16" and 1/32"), without detracting from the excellent mechanical
and physical properties that are imparted to objects formed from PETP moulding compositions.
[0014] The invention is characterized in that, based on the total composition, the moulding
compound contains;
a) 5-15% by weight of at least one of the oxalic acid salts of the group formed by
K3 [Al(C2O4)3], K2 [Mg(C2O4)2] and Rb3 [Al(C2O4)3];
b) 0,1-0,5% by weight of polytetrafluoroethylene (PTFE) having a number average molecular
weight, Mn, higher than 105;
c) 0-40% by weight filler, either fibrous or non-fibrous, in any case at most 10%
by weight glass fibres;
d) additives usual for PETP moulding compositions.
[0015] By usual additives are to be understood here substances such as pigments, mould release
agents, nucleating agents and crystallization accelerating agents, thermal stabilizers
and ultraviolet stabilizers.
[0016] It has been found that the combination of a relatively small amount of the PTFE and
a relatively small amount of flame retardant agent of said group imparts excellent
non-burning properties to unreinforced PETP and has no detrimental effect on its favourable
mechanical and physical properties. More particularly, it has been found that the
presence of a small amount of PTFE not only suppresses the dripping tendency, but
also enhances the effect of the flame retardant agent to such a degree that the moulding
compound containing it will attain the UL 94 VO classification for a thickness of
1,6 mm (1/16") and even for a thickness of 0,8 mm (1/32") while retaining its favourable
mechanical properties, such as impact strength.
[0017] Moreover, the heat distortion temperature of objects from this moulding composition
is found to be at a high level. More particularly, the heat distortion temperature
is higher than that of objects made from a moulding composition which contains an
equally high amount of a halogen-based fire retardant agent in combustion with PTFE.
[0018] The use of a small amount of the selected complex oxalic acid salts in combination
with a small amount of PTFE having a molecular weight Mn higher than 10
5 has been found to be extraordinarily effective in rendering PETP moulding compositions
non-burning up to the maximum attainable UL 94 classification, which effect is especially
surprising in that it is far stronger than in the otherwise yet very much related
PBTP moulding compounds. If in PBTP the same effect is to be obtained as far as incombustibility
is concerned, then said combination or the traditional compounds based on halogen
together with PTFE need be used in larger amounts, resulting in a deterioration of
the mechanical and the physical properties.
[0019] It is preferred that the moulding composition should contain 8-12% by weight of the
complex K
3 [Al(C
2O
4)
3]. Generally, only as little as 0,15-0,3% by weight of PTFE need be incorporated in
the moulding compound. Such as PETP moulding compound meets the classification UL
94 VO for a thickness of 1,6 mm (1/16") and has a Charpy impact strength in accordance
with ISO-R 179 of at least 30 kJ/m
l.
[0020] Preference is given to the use of K
3 [Al(C
3O
4)
3] because of its strong effect and the relatively low price of this salt. For the
preparation of the oxalate complexes reference is made here to the afore-mentioned
Netherlands Patent Application 7 612 884.
-
[0021] The PTFE to be used should have a molecular weight Mn of at least 10
5 PTFE having a lower molecular weight hardly contributes to enhancing the flame retardant
properties of the oxalate complexes and to controlling the tendency to drip of a burning
material.
[0022] Suitable PTFE is commercially available in the form of a powder and as a dispersion
in water. Suitable powders are HOSTAFLON
@TF 1400 and 1740 of Hoechst, TEFLON@ No. 6 of Du Pont de Nemours and suitable dispersions
are FLUON
@GP 1 of ICI and HOSTAFLON
@TF 5032 and 5034 of Hoechst.
[0023] Although the invention mainly envisages providing non-reinforced PETP compositions
that conform to the highest standards of incombustibility, the resulting favourable
properties are maintained if fibrous material is Incorporated into the moulding composition.
By fibrous materials are to be understood here fibres, such as glass fibres, metal
fibres, synthetic fibres and carbon fibres, that are usually employed for reinforcing
polymers.
[0024] Curiously enough, however, it has been found that the addition of more than 10% glass
fibres has a negative influence on the fire retardant properties.
[0025] The favourable fire resitant and flame retardant properties are fully maintained,
however, if into the moulding composition there are incorporated fillers, by which
are to be understood finely divided substances in the form of particles which are
not of a distinctly fibrous nature. The fillers envisaged here do not result in a
higher tensile strength or flexural strength of the material, but they do enhance
its rigidity, which is in contrast with fibrous fillers, which also enhance the tensile
strength and the flexural strength. As examples of suitable fillers may be mentioned
glass spheres, talc, kaolin, wollastonite, chalk, mica, powdered metal and the like,
in an amount of 1―40% by weight, based on the moulding composition. It should be added
that although the mineral wollastonite particles have a somewhat elongated shape,
they do not have the abovementioned distinctly fibrous nature which is characteristic
of, for instance, reinforcing fibres such as glass fibres.
[0026] The effect of the combination of the selected oxalic acid salts and the PTFE in PETP
has been found to decrease with increasing fibre content in the moulding composition.
With the moulding composition containing 30% glass fibres the effect has been found
to decrease considerably. Nor is there any question of extraordinary effectiveness
in PBTP moulding compositions, irrespective of whether or not they contain glass fibres
in a reinforcing amount, which effect however does happen to be found with PETP moulding
compounds containing no or very little fibrous material and must therefore be regarded
as unpredictably favourable.
[0027] The flame retardant effect of the oxalate complexes is at least partly based on the
splitting off of CO
2, as a result of which the supply of 0
2 is screened off. The formation of the protective gas is clearly manifest by the high
degree of foaming on the surface of a burning object of PETP containing these flame
retardants. It would therefore have been obvious for the fire resistance to be promoted
and the dripping of burning material to be resisted by incorporating into the moulding
compound a means which would lead to a chemical bond between the polyethylene terephthalate
molecules or the residues thereof when the material is bruning. A suitable substance
to this end is, for instance, Ca(OH)
21 of which it is known that when used in combination with halogen-containing organic
flame retardant agents it considerably suppresses the dripping tendency of the formed
objects as a result of the increase in effective molecular weight by the coupling
of the polymer chains. According to German Patent Application 2 524 195 the use of
2% by weight of Ca(OH)
2 suffices for a PBTP moulding compound containing a mixture of 996 by weight of decabromodiphenyl
ether and 4% by weight of antimony oxide as flame retardant agent to be rendered non-dripping
and be classed UL 94-SE-O, no mention being made however of the thickness of the test
bars. In the UL 94-SE test no aged material is examined. The results are therefore
generally somewhat more favourable than in the case of the otherwise similar UL 94-VO
test.
[0028] It has now been found, however, that the use of PETP moulding compounds of Ca(OH)
2 in combination with the oxalates as flame retardant agents does not produce any effect
in controlling the dripping of a burning material.
[0029] It is therefore surprising that PTFE, more particularly when it has a molecular weight
Mn higher than 10
s, does appear to be capable of enhancing the effect of said complex salt of oxalic
acid, especially considering that PTFE is extraordinarily inert and will not form
or cause the formation of a chemical bond between the PETP molecules or residues thereof.
[0030] It is also known that not only fibres, such as glass fibres, and Ca(OH)
2, but also other substances, for instance, colloidal Si0
2, sodium acetate and calcium chloride contribute to the effect of flame retardant
agents in thermoplastic polymers. It has been found, however, that PTFE having a molecular
weight Mn higher than 10
6 has an unexpected and extraordinarily strong effect on the action of the oxalate
complex in PETP moulding compositions, whereas colloidal Si0
2, Ca(OH)
2 and sodium acetate are hardly effective as such in combination with said oxalates
in PETP. PTFE having a molecular weight Mn lower than 10
5 practically entirely lacks this favourable effect. It has further been found that
PTFE in the form of very small particles, for instance of 0,1-50 ,um, is just as strongly
effective as when it is used in the form of coarser particles, for instance of 300-500
µm, provided that the molecular weight Mn is chosen higher than 10
5.
[0031] For the preparation of the PETP reference is made to the technical literature. The
molecular weight of PETP suitable to be processed into moulded objects should not
be too low, because in that case the material would have insufficient impact strength.
It is common practice to choose a relative viscosity higher than 1,75. The relative
viscosity is expressed as that of a 1%-solution of the PETP in metacresol at 25°C.
[0032] The PETP may contain relatively small amounts, but preferably not more than 5% by
weight, of other thermoplastic polymers, such as polyethylene, polypropylene, polyamide,
polybutylene terephthalate, and copolymers built up from ethylene terephthalate together
with an ester of terephthalic acid and an other diol, such as 1,3-propanediol, 1,4-butanediol.
[0033] The moulding compounds according to the invention may be prepared by the mixing methods
that are commonly employed for incorporating additives into thermoplastics. Thus,
the oxalate, the PTFE, other additives, if any, and the PETP granulate may be precompounded
or not before being fed to an extruder. After the additives have been mixed with the
molten polymer, the resulting blend is extruded, cooled and processed into granulate
again. Alternatively, the PETP granulate may be coated with the oxaolate and PTFE
in the form of a powder or a dispersion and subsequently be processed or again formed
into a granulate. However, it is also possible for the oxalate to be fed to the polycondensation
mixture in the form of a dispersion in ethylene glycol prior to the polycondensation
process.
[0034] Fillers such as talc and wollastonite may be metered into the feed port of the extruder
in a usual manner at a uniform rate so that they are incorporated into the moulding
compound while properly mixed.
[0035] The material is tested for flammability in accordance with the following procedure.
In conformity with the UL 94 (1979) test method of Underwriter's Laboratories the
material is injection moulded into test bars measuring 127 x 12,7 x 1,6 mm (1/16")
and 127 x 12,7 x 0,8 mm (1/32"). In each test the average value is determined of the
results of 5 bars conditioned for 48 hours at 23 ± 2°C and 50% R.H. and that of 5
bars artificially aged by exposure for 7 days to air of 70°C.
[0036] Each bar is vertically suspended and a flame is kept against its lower end for 10
seconds. This procedure is repeated when the flaming of the test bar ceases.
[0037] For a material to be classed UL 94-VO and VI, respectively, it shall comply with
the following requirements:
a) none of the 10 test bars shall burn for more than 10 seconds (30 seconds) after
the first and the second removal of the flame;
b) the total flaming combustion shall not exceed 50 seconds (250 seconds) for the
10 flame applications for each set of 5 test bars;
c) none of the test bars shall drip flaming particles that ignite absorbent surgical
cotton located 300 mm below the test bar;
d) none of the test bars shall display glowing combustion which persists beyond 30
seconds (60 seconds) after the second removal of the test flame. (All bracketed values
for V1).
[0038] The LIO (Limiting Oxygen Index) of the moulding compounds is measured in conformity
with ASTM-D 2863 with the aid of a tester made by Stanton Redcroft, Great Britain.
The LOI value is defined as the oxygen concentration expressed as a percentage of
a mixture of oxygen and nitrogen that will just support combustion of a vertically
clamped test specimen whose top is contacted with a flame.
[0039] The viscosity of the polyester of the moulding compound is determined in accordance
with the following procedure.
[0040] The relative viscosity of the PETP of the moulding compound is measured with a solution
of 1 g PETP in 100 ml of a mixture of trichlorophenol and phenol, weight ratio 72:100,
at 25 ± 0,05°C with the aid of a Ubbelohde capillary viscometer. The value found is
converted into the value which would have been found in m-cresol, use being made of
the following formula:
[0041] The relative viscosity of a PBTP moulding compound is measured in a similar way in
a solution of 1 g PBTP in 100 ml m-cresol at 25 ± 0,05°C.
[0042] The determination of the molecular weight Mn of the PTFE is carried out in accordance
with the method described by Suwa, Takehisa and Machi in J. Appl. Pol. Sci. 17 (1973);
pp. 3253-3257.
[0043] This method is based on the relationship between the molecular weight Mn and heat
of crystallization, the latter being measured by differential thermal analysis.
[0044] This relationship is as follows:
AHC in cal/g
[0045] The invention will be further described in the following examples.
Example 1
[0046] In this example the flame retardant properties of a PETP moulding composition containing
a complex salt of oxalic acid both without and in combination with PTFE are compared
with the flame retardant properties of a PETP moulding composition containing an equal
amount of a commonly employed halogen-containing flame retardant without and in combination
with a corresponding amount of PTFE. As halogen-containing flame retardant there is
used a mixture of 50% by weight of decabromodiphenyl ether (DBDE) and 50% by weight
of antimony oxide, which mixture is known to be a particularly effective flame retardant
for thermoplastic polyester.
[0047] The following materials are processed:
Granulate of PETP having a relative viscosity of 1,98.
[0048] Powdered potassium-aluminium oxalate, K
3(Al(C
2O
4)
3], (K.Al.ox). For details about the preparation reference is made to Netherlands Patent
Application 7 612 884.
[0049] Decabromodiphenyl ether, C
12Br
10O (DBDE).
[0050] Antimony trioxide, Sb
2O
3:
[0051] Polytetrafluoroethylene, type Hostaflon®TF 1740 of Hoechst AG, molecular weight Mn
about 3.2 . 10
6, average particle size 40 µm (PTFE).
[0052] In a rotary mixer the PTFE powder is distributed over the surface of the PETP granulate.
This granulate and the K.Al.ox are fed at a uniform rate to a twin scew extruder of
the Werner und Pfleiderer type, ZDSK 53.
[0053] The cylinder temperature is set to 265°-270°C.
[0054] The melt is extruded and re-processed into granulate. Subsequently, the usual amounts
of mould release agent and crystallization accelerator are added.
[0055] This granulate is injection moulded into test bars having a thickness of 0,8 mm (UL
1,32") or 1,6 mm (1/16") at a Stübbe injection moulding machine of the SKM 51 type.
[0056] The PETP moulding compound containing DBDE and the test bars thereof are made in
the same manner.
[0057] The results of the UL 94 and ASTM D2863 test procedures are summarized in Table 1.
[0058] The above results show that a PETP moulding composition containing as little as 10%
by weight of the complex potassium-aluminium oxalate and 0,25% by weight of PTFE of
Mn 3,2 108 already meets the highest UL flame retardance requirement for the thinnest
test bars, which is in contrast with a PETP moulding composition into which for the
oxalate an equally high amount of the known effective flame retardant combination
DBDE and Sb
2O
3 is incorporated.
[0059] In particularly appears that the PTFE moreover enhances the flame retardant effect
of the oxalate after aging of the moulding. But PTFE reduces the flame retardant effect
of the DBDE-Sb
2O
3- combination in PETP moulding compounds.
Example 2
[0060] In this example the effect of PTFE as anti-dripping agent in a PETP moulding composition
and the complex potassium-aluminium oxalate as flame retardant is compared with the
effect of other anti-dripping agents known to be used for polyester.
[0061] In the manner described in Example 1 a granulate is prepared from PETP moulding compositions
containing 10% by weight K
3 AI(C
2O
4)
3. Uniform distribution over the surface of the granulate leads to the incorporation
therein respective of:
[0062] This Si0
2 is added together with the oxalate.
[0063] The granulate thus treated is formed into test bars in the manner described in Example
1. The moulding composition containing PTFE is prepared in accordance with Example
1.
[0064] The test in accordance with the UL 94 procedure leads to the results summarized in
Table 2.
[0065] The above results show that the effect of the combination of 10% potassium-aluminium
oxalate and 0,25% PTFE as fired retardant agent in PETP is far stronger than that
of the combination of 10% potassium-aluminium oxalate with other agents known in themselves.
Moreover, in contrast with other agents PTFE is found to considerably enhance the
flame retardant effect of the oxalate.
Example 3
[0066] In this example a comparison is made between the most important mechanical properties
of non-filled PETP moulding compounds which contain as flame retardant agent the oxalate
combined with PTFE and those of PETP and PBTP moulding compounds which contain as
flame retardant agent the mixture of decabromodiphenyl ether - antimony oxide (weight
ratio 1:1) and PTFE.
[0067] In a rotary mixer the PETP granulate is subject to polycondensation under vacuum
and at a granulate temperature of 220°C to the viscosity mer oned in Table 3.
[0068] The test bars are obtained in the manner described in Example 1.
[0069] The PBTP starting polymer has a relative viscosity of 2,10.
[0070] The results are listed in Table 3.
[0071] The above results show that a non-filled PETP moulding composition containing as
little as 10% by weight of potassium-aluminium oxalate and 0,25% by weight of PTFE
not only has excellent fire retardant properties, as is shown in Table 1, but also
very favourable mechanical properties.
Example 4
[0072] This example is concerned with the influence of such fillers as talc, wollastonite
and glass fibre on the fibre retardant properties of a PETP moulding composition containing
as fire retardant agent the complex potassium-aluminium oxalate and PTFE.
[0073] The fillers are metered into the feed port of the extruder. For the preparation the
same conditions apply as mentioned in Example 1, except that the PETP starting material
has a relative viscosity of 1,65.
[0074] The test results are listed in Table 4.
[0075] This example shows that as little as 8-10% by weight of potassium-aluminium oxalate
combined with 0,25% by weight of PTFE suffices for PETP moulding compounds containing
25-30% by weight of non-fibrous fillers to meet the UL 94-VO requirements for 1,6
mm (1/16"). This classification is not attained if the filler is a glass fibre.
Example 5
[0076] In this example the influence is investigated of the molecular weight Mn of the PTFE
on the fire retardant of a PETP moulding composition. The moulding compositions are
prepared in the manner indicated in Example 1.
[0077] The results are summarized in Table 5.
[0078] The following PTFE types are tested:
A) Polymist®F5A, manufactured by Allied Chemicals.
powered, Mn about 2,3.104.
B) Fluon®GP1, manufactured by lCl.
dispersion in water, Mn about 8,5.106, particle size 0,2-0,5 µm.
C) Hostaflon®TF 1740, manufactured by Hoechst.
powdered, Mn about 3,2.106, particle size 40 µm.
[0079] The above results show that the afore-mentioned influence on fire retardant properties
is only found with PTFE types having a molecular weight (Mn) higher than 10
5.